Growth and ion uptake by wheat supplied nitrogen as nitrate, or ammonium, or both

The effects of concentration and source (NH4, NO3, and NO3 plus NH4) of added N on the rate of growth, final yield, and content and rate of intake of N, P, K, Ca, Mg and S by wheat seedlings were evaluated. Rate of growth in dilute liquid cultures differed among the N sources giving yields relative to those of the all-NO3 system of 92 per cent for the all-NH4 system, and of 154 per cent for the NO3 plus NH4 system. At low rates of NH4 intake in the all-NH4 systems growth rates were equal to or slightly better than those of plants supplied equivalent concentrations of NO3. Rates of NH4 intake exceeding 100 μmole g−1 h−1 resulted in reduced growth rates and incipient NH4 toxicity. Yields at 95 per cent of maximum resulted with steady-state N concentrations of 80 μM in all NO3 systems, 30 μM NH4 in all-NH4 systems, and in combined source systems when 200μM NO3 plus 30 μM NH4 were supplied. The rate of N intake and plant protein content, were maximal when both NO3 and NH4 were supplied. Increasing rates of NO3 intake were associated with increases in the rates of Ca, Mg, and K intake; but with increasing rates of NH4 absorption, intake of Ca and Mg decreased. The yield and growth rate enhancement observed from the addition of low concentrations of NH4 to cultures supplying adequate NO3 is suggested to result from the reduced energy requirement for utilization of NH4, as compared to NO3 in protein synthesis and from the increased photosynthetic capacity of the higher-protein NH4-fed plants. In the all-NH4 systems the maximum attainable growth rate was limited by NH4 toxicity; whereas in the all-NO3 systems the rate of NO3 reduction was limiting.